The manufacturing of electronic devices often involves the attachment of a semiconductor die onto a substrate prior to final packaging of the electronic devices. Before a semiconductor die is attached to the substrate having a metallic surface, such as a lead frame, the substrate or lead frame is typically pre-heated in a heat tunnel in order to create conditions which are conducive to die attachment. The heat tunnel has heaters to pre-heat the lead frame to a temperature above the melting point of soft solder to enable the solder to become the medium for die attachment. Solder may be dispensed by way of a length of solder wire that is lowered onto a pre-heated lead frame and which melts upon contact with the pre-heated lead frame. The lead frame is then transported to a bonding zone within the heat tunnel whereat the semiconductor die is bonded. Finally, the lead frame is cooled to solidify the solder to complete the bond. Conventional soft solder die attach applications employ forming gases, which may contain 5-15% hydrogen, to impede oxidation of the lead frame during such heating process.
Fluxless soldering is the most preferred method for die attachment and is widely used in industry. Amongst various fluxless reflow and soldering methods, the use of hydrogen as a reactive gas to reduce oxides on substrates is especially attractive because it is a clean process and is compatible with an open and continuous production line. Therefore, fluxless soldering which is carried out in the presence of hydrogen has been a technical goal for a long time. One approach has been to employ forming gas comprising 5-15% hydrogen in a nitrogen carrier gas to exhaust air, especially oxygen, from the heat tunnel. The oxygen level in the heat tunnel is maintained at below 50 ppm to protect the lead frame from oxidation. Furthermore, the forming gas can be used to reduce copper oxide that is present on the surface of the lead frame to improve solder wettability.
The heat tunnel would usually be full of the forming gas mentioned above. However, for soldering processes used in die attachment, a major limitation is the inefficient and slow reduction rate of metal oxides, especially in respect of solder oxides. This inefficiency of hydrogen is attributable to the lack of reactivity of hydrogen molecules at low temperatures. While active hydrogen is important for reducing oxide, highly reactive radicals such as mono-atomic hydrogen can be formed only at high temperatures. For instance, the effective temperature range for reducing copper oxide is above 350° C., and even higher temperatures (of more than 450° C.) are necessary to effectively reduce solder oxide. Usually, relatively limited amounts of hydrogen gas can be activated in a conventional heat tunnel of a soft solder die bonder. Therefore, it would be desirable to be able to generate highly reactive hydrogen, and thus decrease the required amounts of hydrogen concentration and processing temperature for effective reduction of oxides such as solder oxide.
Moreover, due to several open windows in the heat tunnel for process operations, such as solder dispensing, spanking and die bonding, air often diffuses and blows as a tourbillion into the heat tunnel. This makes it challenging to achieve an oxygen-free environment in the heat tunnel in order to achieve a high level of anti-oxidation for good soldering. Without effective reduction of solder oxide, the solder oxide which is created will result in void and die tilting issues during die attachment, and would induce reliability problems.
A further negative trend is that more and more low-end lead frames with degraded solder wettability are being used. These lead frames are more prone to copper oxide formation on their surfaces, which prove challenging when using traditional forming gas to impede oxidation.
For the above reasons, the effectiveness of the reducing gases that have been conventionally used should be improved.